**3.3 Types of digester**

During the last century a number of different types of flows in simple digester have been developed and they can be of the following kinds: (1) batch flow, (2) continuous flow, (3) continuously expanding, (4) plug flow, and (5) contact flow. The conventional digesters are those utilized to process liquid raw materials with a high content in solids, also called rural digesters, the fermentation chamber having a volume below 100 m3. Conventional digesters

2. The objective of treating the organic waste (the production of energy and/or organic

The amount of manure fed into a digester each day has an important effect on its operation. This is measured by volume added in relation to the volume of the digester, but the actual quantity fed to the digester also depends on the temperature at which the digester is maintained. In order to determine the unit size of a biogas unit, the following mathematical

 Digester size (m³) = Daily feed-in (m³ day-1) × Retention time (day) (1) The digester size can be defined as the total size of the biogas unit, which includes the effective size of any volume occupied by the fermented material and the volume of gas storage. Size of the daily feed-in is the size of a mixture of dung with water added to the digester once daily or several times and the average concentration of total solids of 10%, where mixing the organic wastes with water depends on its water content. In the case of wet animal wastes, such as manure the proportion of mixing is 1:1. Generally, Storage capacity has to be calculated by average live weight of animals kept in husbandry systems, amount

In order to plan a biogas plant and to design a digester, several design parameters must be determined which are: ratio of gathered waste from manure canals to total waste, number of cows in farm, amount of manure produced by a cow which is usually 1.8 m3 cow-1 month-1, quantity of daily liquid organic matter deposition into the digester, hydraulic retention time, density and quantity of daily dry organic matter deposition into the digester, and digester load which is usually 2-4 kg m-3 day-1. The aforementioned design parameters are used to determine the total volume of the materials that are intended to be stored in the tank and are equal to the internal volume of the tank. Additionally, the designer should take into consideration that a part of the tank (about 10%) is empty and the substrates should not fill it, because it is the place where the gas will accumulate. Even in case of designing other storage tanks (e.g. liquid organic matter tank) it is required to leave 10% of the tank volume

During the last century a number of different types of flows in simple digester have been developed and they can be of the following kinds: (1) batch flow, (2) continuous flow, (3) continuously expanding, (4) plug flow, and (5) contact flow. The conventional digesters are those utilized to process liquid raw materials with a high content in solids, also called rural digesters, the fermentation chamber having a volume below 100 m3. Conventional digesters

5. Air temperature in the region and wind direction throughout the different seasons 6. The training level of the staff on farm and home regarding operation of biogas units

The size of a biogas unit depends on several factors, which are:

3. Demand of natural gas and consumption pattern 4. On-site nature of the soil and the level of ground water

1. The amount and type of organic waste to be disposed in the digester

of added water, periods of no fertilization of crops, and the animal species.

**3.2 Size of biogas unit** 

fertilizer)

equation must be achieved:

empty.

**3.3 Types of digester** 

are installed without any type of mechanism to reduce the retention time during which the biomass remains inside are predominant; these systems are fed discontinuously and known as discontinuous-flow i.e. batch digesters, or fed periodically and known as continuous-flow digesters.

Batch digesters are loaded at once, maintained closed for a convenient period, and the organic matter is fermented and then unloaded at a later time. It is quite a simple system with small operational requirements. Installation can be made in an anaerobic tank or in a series of tanks, depending on the biogas demand, availability and amount of raw materials to be utilized. Batch flow is most suitable for dry organic matters (solid materials), e.g. solid vegetable waste. This type of biowastes is fed into the digester as a single batch. The digester is opened, digestate is removed to be used as biofertilizer and the new batch replaces the digestate. The tank is then resealed and ready for operation. Depending on the waste material and the operating temperature, a batch digester will slowly start producing biogas and increase the production with time and then drop-off after 4 to 8 weeks. Batch digesters are therefore best operated in groups, so that at least one digester is always producing biogas.

Continuous digesters are usually requiring daily loading and residue management. The process is referred to as continuous since to every daily load corresponds a similar volume load of fermented material. The biomass inside the digester moves through by the difference in hydraulic heat, between the substrate entering the digester and the digestate coming out when unloading. Each load requires a retention time, usually between 14 to 40 days. Continuous digesters can have their retention period reduced by the introduction of agitation and heating. The disadvantage of these models is that the raw material needs to be diluted. The great advantage of these digesters over the batch type is that a single unit allows a continuous supply of biogas and biofertilizer and the continuous treatment of small amounts of waste (Florentino, 2003). Biogas production can be accelerated by continuously feeding the digester with small amounts of waste daily. If such a continuous feeding system is used, then it is essential to ensure that the digester is large enough to hold all the material that will be fed into the digester in the whole digestion cycle. One key issue is to implement two digesters, i.e. accomplishing the biodegradation of the organic waste through two stages, with the main part of the biogas is being produced in the first stage and the second stage serves as finishing stage of the digestion at a slower rate.

Regarding the continuously expanding flow, the digester starts one third full and then filled in stages and later emptied. Concerning the plug flow, the wastes are added regularly at one end and over-flows the other. In the contact flow, a support medium is provided.

Two simple biogas digester designs have been developed, the Chinese fixed dome digester and the Indian floating cover biogas digester. The digestion process is the same in both digesters but the gas collection method is different in each. In the Indian-type digester, the water sealed cover of the digester rises as gas is produced and acts as a storage chamber, whereas the Chinese-type digester has a lower gas storage capacity and requires efficient sealing in order to prevent gas leakage. Both have been designed for use with animal waste or dung. Additionally, there are also Philippine and Sri Lankan digesters.

#### **3.3.1 Indian digester**

The Indian-type digester (Fig. 1) basically is comprised of a cylindrical body, gasometer, feed pit and outlet pit (Florentino, 2003). The digester is made using burnt-clay bricks and

Biogas Plant Constructions 347

The Chinese-type model digester (Fig. 2) is comprised of a cylindrical body, two spherical domes, inlet pit, outlet pit and an inspection opening (Florentino, 2003). The digester is made using cement and bricks and it is a permanent structure. Just as in the Indian digester

The biogas is collected in the upper chamber and the waste decomposes in the lower chamber. If the gas pressure exceeds the atmospheric pressure (1 bar) and there is no gas extracted from the dome, then the rot substrate squeezed from the reactor into the filled pipe, but often in the pool of counterpoise. If the produced gas is more than the up used gas, then the slime level will increase. If the up used gas is more than the produced gas during the gas extraction, then the slime level will sink and the rot slime will flow back. The volume of the counterpoise pool must be huge so that the repressed rot substrate can be digested at the highest gas volume. The gas pressure is not constant in the practice. It increases with the quantity of the stored gas. The gas must be regularly produced; therefore the gas pressure

Owing to the fact that the biogas dome digesters are completely buried underground, the fermentation temperature should be under a day/night temperature change, only in a tolerance range from about ± 2 ºC. The difference between summer and winter is large and is subject to the climate zone. The biogas dome digester can be provided with stir. In small family household units, a mix concoction for the biogas dome digester is installed. Different building and construction forms of biogas dome digesters were proved for the Chinese

this has two drains to feed waste and to collect the composted waste.

organizer or the swimming gas repository room is important.

digesters; so that there is a big number of building methods are used.

Fig. 2. Chinese-type digester (Florentino, 2003)

**3.3.2 Chinese digester** 

cement. The cylindrical dome is made of metal sheets and moves up and down as it stores and releases the biogas. The digester is operated in continuing method and often vertically, almost cylindrical built. The putridity space filled the ground and it has a dividing wall. This dividing wall improves and holds back the fresh slime gush again through short way. The gas is gathered in floating gas lock. The steel gas lock is provided with stir elements. The periodic destruction of swimming layer is performed using the manual stirring of gas lock. The requested gas pressure arises from the heaviness of the swimming gas lock. The gas pressure can basically be changed in the practice by putting things on the gas lock.

This type is suitable for the homogeneous materials, as for the animals' excrements that do not tend to build sinking layers. The green waste must be split. If it is mixed with huge allotments, then it will threat the digester with blockage. Generally, there are several designs of Indian digesters, thereof: floating gas holder type biogas plant (KVIC model), Deenbandhu model, and Pragati model. The KVIC model is composite unit of a masonry digester and a metallic dome, where the maintenance of constant pressure by upward and downward movement of the gas holder. The Deenbandhu model consists of segments of two spheres of different diameters joined at their base, where this model requires lower costs in comparison to KVIC model. The Pragati model is a combination of Deenbandhu and KVIC designs, where the lower part of the digester is semi spherical with conical bottom and the floating drum acts as gas storage.

Fig. 1. Indian-type digester (Florentino, 2003)

cement. The cylindrical dome is made of metal sheets and moves up and down as it stores and releases the biogas. The digester is operated in continuing method and often vertically, almost cylindrical built. The putridity space filled the ground and it has a dividing wall. This dividing wall improves and holds back the fresh slime gush again through short way. The gas is gathered in floating gas lock. The steel gas lock is provided with stir elements. The periodic destruction of swimming layer is performed using the manual stirring of gas lock. The requested gas pressure arises from the heaviness of the swimming gas lock. The gas pressure can basically be changed in the practice by putting things on the gas lock.

This type is suitable for the homogeneous materials, as for the animals' excrements that do not tend to build sinking layers. The green waste must be split. If it is mixed with huge allotments, then it will threat the digester with blockage. Generally, there are several designs of Indian digesters, thereof: floating gas holder type biogas plant (KVIC model), Deenbandhu model, and Pragati model. The KVIC model is composite unit of a masonry digester and a metallic dome, where the maintenance of constant pressure by upward and downward movement of the gas holder. The Deenbandhu model consists of segments of two spheres of different diameters joined at their base, where this model requires lower costs in comparison to KVIC model. The Pragati model is a combination of Deenbandhu and KVIC designs, where the lower part of the digester is semi spherical with conical bottom

and the floating drum acts as gas storage.

Fig. 1. Indian-type digester (Florentino, 2003)
